Such heaters are used, for example, to heat tools, machine parts and apparatuses, especially plastic injection nozzles. However, the use of such heaters is not limited to this purpose; for example, it is possible to bring fluids to a desired temperature by passing them though this type of heater. As an alternative, stagnant fluids or an expanding wax can also be brought to a desired temperature.
Especially popular embodiments of such heaters are those with a cylindrical geometry.
The prior-art heaters comprise, as a rule, two jacket surfaces, which are arranged at spaced locations from each other and between which at least one heat conductor is arranged in an insulated manner. The heat conductor may be in the form of a band or wire or another resistance wire. The “inner jacket surface” is always defined within the framework of the present application as a jacket surface that is in contact with the component to be heated or is formed by the component to be heated itself; the “outer jacket surface” correspondingly faces away from the component to be heated.
It is known from the state of the art, e.g., DE 103 33 206 B4, to arrange the heat conductor on an insulating, especially tubular, coil body or carrier, which is then arranged between the boundary surfaces.
It is also desirable at the same time that the heating output of the heat conductor be transmitted to the inner boundary surface as directly and homogeneously as possible and that the most space-saving and compact embodiment possible is obtained. This objective can be accomplished with the use of a coil body or carrier to a limited extent only, because, on the one hand, it must have the necessary mechanical stability and, on the other hand, it must have sufficiently high manufacturing tolerances to permit installation of the coil body or carrier in the electric heater despite deviations in its roundness from the ideal shape, its deflection and possible variations in diameter over the length of the body. In particular, its internal diameter must additionally also have so much clearance that deflection and circularity error of the inner jacket surface are absorbed. Narrow gaps may form at least locally between the coil body or carrier, on the one hand, and the inner jacket surface, on the other hand, which prevent good and homogeneous transmission of the heat output and lead to inhomogeneities during the compaction of the heating element.
Even if filling with a powder or granular material is performed, it frequently happens that an air cushions will remain. If a subsequent compaction is then performed on a setting plug, a jacket surface may form folds, which makes the heater unfit for use or impairs the heat transfer to the object to be heated.